(The Hill) President Donald Trump said Monday he thinks last-minute pardons from former President Joe Biden are “void” due to them being signed with an autopen.

Biden pardoned multiple political enemies of the president preemptively, including members of the House panel that looked into the Jan. 6, 2021, U.S Capitol riots, members of his family, Anthony Fauci and Gen. Mark Milley, right before Trump’s January inauguration.

“The ‘pardons’ that Sleepy Joe Biden gave to the Unselect Committee of Political Thugs, and many others, are hereby declared VOID, VACANT, AND OF NO FURTHER FORCE OR EFFECT, because of the fact that they were done by Autopen,” Trump said on social media early Monday.

Here’s what you need to know about presidential use of an autopen, the tool Trump alleged Biden used for the pardons:

What are they?

According to NPR, autopen is a generic name given to a machine that uses real ink for signature reproduction. Included with the machines is a pencil or pen-holding arm with the ability to recreate, on paper beneath it, the programmed signature, per the outlet.

Do they have a history of use by presidents?

Former President Obama used an autopen multiple times during his presidency, including to sign spending and transportation funding bills. In 2011, the former president even faced questioning from a House Republican over his autopen usage in signing a Patriot Act extension.

NBC News also reported that Biden, like his former boss, has his own history of autopen usage.

Is their use legitimate?

A memorandum opinion from the Department of Justice’s Office of Legal Counsel during former President George W. Bush’s time in the White House states that a president can sign a bill via ordering a person below them to “affix the President’s signature to it.”

The BBC also reported autopen-signed pardons would not be invalidated by anything under U.S. law, according to legal experts the outlet talked to.